Adhesive compositions, particularly conductive adhesives, have recently gained great popularity in the microelectronics industry. These adhesives are used for a variety of purposes with one of the more prominent uses being the attachment of semiconductor dies or chips, to a substrate, or other support medium. A common problem associated with these adhesives is their rigidity or inflexibility. When used to attach chips to a metallic lead frame, stress will be placed upon the adhesive during thermal cycling, due to the dissimilar thermal expansion coefficients of the adhesive and the metal frame. Such stress often leads to adhesive failure or fracture of the silicon die chip. Another problem in regard to such adhesives is ionic contamination by alkali metal (e.g. sodium and/or potassium) and chloride species which, under certain circumstances, can lead to corrosion of the electrical leads in the vicinity of the adhesive.
Recently, in U.S. Pat. No. 4,557,860 to A. L. DiSalvo et al., an adhesive system was described which comprised, as essential ingredients, an epoxy resin, a soluble polyimide resin, and a monoepoxy diluent. Such an adhesive is curable by use of conventional epoxy curing agents. If conductive adhesives are desired, a suitable conductive filler can be added to such compositions. However, introduction of such fillers can cause the adhesive to be contaminated with ionic species undesirable for use in many electronic applications.
Other epoxy modifiers, notably carboxy terminated butadiene acrylonitrile copolymers, have been used as epoxy "toughening agents", to increase the flexibility and peel strengths of the adhesives, (eventually leading to chip fracture) but these, too, have been unable to allow the adhesive to relieve stress thermal during cycling, which is indicative of the unsuitability of the adhesive in normal use. The adhesives were also prone to contamination with undesirable ionic species.
Further, most epoxy adhesives with added toughening agents have high viscosities and require the addition of diluents to reduce these viscosities to levels for normal use. Such diluents often are ionically impure, making the adhesives less desirable for use in electronic applications.
There exists a real need for high purity, low viscosity adhesives which are stable toward thermal cycling for use in die attach applications.